The present invention relates to geogrids, which are plastics grid structures made from stretching a starting material having a pattern of holes formed therein, the structures having mesh openings with molecularly-oriented strands on at least two opposite sides, junctions in the structures being at least partly molecularly-oriented. Geogrids are described in, for instance, U.S. Pat. No. 4 374 798 and EP A-O 374 365.
A uniax geogrid can be considered as a grid formed by parallel, spaced, oriented strands interconnected by interconnecting elements, the interconnecting elements and the respective parts of the strands, which can be loosely termed junctions, forming bars at right angles to the strands. The starting material can be considered as formed of spaced, parallel, continuous longitudinal elements and between the longitudinal elements, interconnecting elements interconnecting the longitudinal elements. The holes are normally on a square or rectangular grid.
As the market acceptance of the uniax geogrid has increased, so has the demand for geogrids of greater strength, and for geogrids having improved economy ratings. HDPE is the preferred raw material for long term applications for uniax geogrids and the Governments of Britain (Agreement Certificate) and the Federal Republic of Germany (Bautechnik Certificate) have issued certificates for such geogrids, which acknowledge that in numerous construction applications they will have a useful life of 120 years. Typical dimensions for uniax geogrids, measured between centres of transverse bars, are 114 mm to 160 mm. Such geogrids exhibit an economy rating in the range of 100 to 120 kN perm m per kg of resin per square metre, and the overall stretch ratio during production is usually in the range of 4.5:1 to 5.0:1. It is possible to increase the economy rating by lengthening the aperture which forms the zones to be stretched, but this results in greater spacing of the transverse bars; a spacing of say 250 mm is considered to be the upper limit in usages such as embankments and soil retaining walls Greater strength can be achieved by increasing the starting material thickness, e.g. up walls. to 6 mm thickness. 6 mm thick starting sheets of high density polyethylene can produce a strength of 110 kN per metre width, but only exhibit an economy rating of 100 kN per m per kg per m.sup.2.
There is a strong market demand for geogrids having high strength and a high economy rating.
Apart from the foregoing, it is desirable to be able to increase the overall stretch ratio and the amount of orientation passing through the junctions without a marked tendency for the junctions to split along the centre lines of the strands or in any part of the junctions.